The present invention relates generally to solder flux compositions, and more particularly to solder flux compositions useful for attaching solder to a substrate or motherboard.
Various solder flux compositions are used in the soldering of electronic components, circuits, equipment and the like so as to improve the efficiency and quality of the soldering operation and to improve the long-term reliability of solder joints. Solder flux compositions are often designed to react with or dissolve metal oxides and impurities commonly present on the surfaces being soldered, and to coat the cleaned surfaces to protect them against oxidation.
The use of solder flux compositions in soldering operations that involve devices having finer features (about 1.0 mm or smaller), such as fine-pitched Ball Grid Arrays (BGAs), places great demands both on the solder flux composition itself and on its method of application. Because of the dimensions involved, devices of this type have very small tolerances for error in terms of the placement of a solder joint. Consequently, if the solder migrates even slightly from its original intended position on the substrate during reflow, electrical bridging can occur between neighboring solder joints, thereby resulting in a defective product.
In some applications, the use of a stencil during the application of a solder flux composition can help to minimize bridging problems by limiting the area to which the solder flux composition is applied, thereby limiting the area over which solder migration tends to occur. However, the use of a stencil as the primary means to control solder migration is not equally effective for all solder fluxes. In particular, while many solder flux compositions have relatively high viscosities at room temperature, some solder flux compositions become substantially less viscous at higher temperatures, and thus tend to migrate from their original placement during reflow. In soldering operations that employ solder flux compositions of this type, the use of a stencil as the primary means to control solder migration with these solder flux compositions may be largely ineffective.
The use of a stencil as the primary means to control solder migration may also be ineffective in soldering operations that involve items, such as fine-pitched BGAs, that have very small features. This may be the case even if the solder flux composition retains a relatively high viscosity at reflow temperatures since, in applications of this type, even small migrations of the solder flux composition from its original position may result in bridging problems. For example, many solder flux compositions perform adequately on medium-pitched substrates (that is, substrates in which the pitchxe2x80x94that is, the distance between the midpoints of adjacent bond pads on the substratexe2x80x94is about 1.27 mm, a standard pitch for many flip-chip BGAs), but result in substantial bridging on finer pitched substrates (for example, on substrates having a pitch of 1.0 mm or smaller). Unfortunately, the ongoing trend in the art toward further miniaturization of device features has resulted in increasing usage of finer pitched substrates, thus creating an increasing need for flux compositions that are more effective at limiting solder migration.
A further issue in the art arises from the desire to replace lead-based solders with lead-free solders. In many applications, the use of lead-free solders is not only desirable, but has become a requirement. Unfortunately, it has been found that many solder flux compositions which perform adequately with lead-based solders do not perform well with lead-free solder compositions (especially on finer-pitched substrates), frequently resulting in substantial bridging problems.
There is thus a need in the art for a solder flux composition, and for a method of applying and using the same, that can be used for ball attach operations on fine-pitched BGAs or in other solder operations involving small feature sizes. There is also a need in the art for a solder flux composition which minimizes the bridging problems that can occur as a result of the migration of solder flux compositions, especially on fine-pitched substrates. There is further a need in the art for a solder flux composition which performs well with lead-free solders. These and other needs are met by the compositions and methodologies described herein.
In one aspect, a solder flux composition is provided herein which comprises active ingredients and a carrier. The composition undergoes a phase separation at a temperature greater than about 100xc2x0 C., preferably greater than about 150xc2x0 C., more preferably greater than about 175xc2x0 C., and most preferably greater than about 200xc2x0 C., to form at least a first phase and a second phase, wherein the active ingredients are disposed primarily in said first phase, and wherein the carrier is disposed primarily in said second phase. The solder flux composition preferably comprises active ingredients, a masking agent, a carrier, and a solvent. The active ingredients typically comprise a carboxylic acid, and preferably comprise phenylsuccinic acid. The masking agent preferably comprises 2-ethylimidazole. The carrier preferably comprises polyethylene glycol, and the solvent preferably comprises a polypropylene glycol butyl ether such as polypropylene glycol monobutyl ether, or tripropylene glycol.
In another aspect, a method for reflowing solder is provided. In accordance with the method, a substrate is provided, which may be, for example, a medium or fine-pitched BGA. A solder flux composition (which may be of the type described above) is then applied to the substrate. The solder flux composition comprises active solder flux ingredients disposed in a carrier. The substrate may comprise a plurality of bond pads, each of which may have a metallization layer disposed thereon, and the solder flux composition may be selectively applied to the substrate with the aid of a stencil such that the solder flux composition is disposed primarily over said plurality of bond pads. A solder is then applied to the substrate that has been treated with the solder flux composition, and the solder is reflowed. During reflow, which preferably involves the step of heating the solder to a temperature greater than 200xc2x0 C., the solder flux composition undergoes a phase separation. Preferably, this phase separation results in the production of a first liquid phase which consists principally of the carrier, and a second phase which consists principally of the active ingredients of the solder flux composition.
In yet another aspect, a method for preparing a substrate, such as a fine-pitched BGA, for a ball attach operation is provided. In accordance with the method, a substrate is provided which has a first surface with a plurality of bond pads disposed thereon. A portion of a solder flux composition, which comprises a solder flux disposed in a carrier, is then applied to each bond pad, thereby forming a treated bond pad. Solder is then applied to the treated bond pad and is reflowed. The solder flux composition is adapted to undergo a phase separation during the reflow of the solder.
In still another aspect, a solder flux composition is provided which comprises phenylsuccinic acid, ethylimidazole, polyethylene glycol, at least one alkylene glycol butyl ether, and a thixotropic agent, and wherein the amount of ethylimidazole present in the solder flux composition is less than 5.4% by weight, based on the total weight of the solder flux composition. The at least one alkylene glycol butyl ether may comprise one or both of polypropylene glycol butyl ether and tripropylene glycol butyl ether. The amount of ethylimidazole present in the solder flux composition is preferably within the range of about 2.0 to about 5.2%, more preferably within the range of about 3.5 to about 5.0%, and most preferably within the range of 3.7 to 5.1% by weight, based on the total weight of the solder flux composition. The amount of phenylsuccinic acid present in the solder flux composition is preferably within the range of about 2 to about 20%, more preferably within the range of about 8 to about 16%, and most preferably within the range of about 10 to about 15.5% by weight, based on the total weight of the solder flux composition. The amount of polyethylene glycol present in the solder flux composition is preferably within the range of about 1 to about 20%, and more preferably within the range of about 3 to about 16% by weight, based on the total weight of the solder flux composition. The amount of the at least one alkylene glycol butyl ether is preferably within the range of about 50 to about 80%, and more preferably within the range of about 60 to about 70% by weight, based on the total weight of the solder flux composition. The amount of thixotropic agent present in the solder flux composition is preferably within the range of about 2 to about 20%, and more preferably within the range of about 3 to about 15% by weight, based on the total weight of the solder flux composition. The thixotropic agent is preferably a wax such as beeswax.
In one particular embodiment, the amount of polyethylene glycol present in the solder flux composition is within the range of about 10 to about 20% by weight, based on the total weight of the solder flux composition, and the polyethylene glycol has an average molecular weight within the range of about 600 to about 1200 g/mol.
In another particular embodiment, the amount of polyethylene glycol present in the solder flux composition is within the range of about 10 to about 20% by weight, based on the total weight of the solder flux composition, and the polyethylene glycol has an average molecular weight within the range of about 500 to about 1500 g/mol, and preferably about 900 g/mol. In yet another particular embodiment, the solder flux composition comprises about 2 to about 20% phenylsuccinic acid, about 2.0 to about 5.2% ethylimidazole, about 1 to about 20% polyethylene glycol, about 50 to about 80% of at least one alkylene glycol butyl ether, and about 2 to about 20% thixotropic agent, wherein all percentages are percentages by weight, based on the total weight of the solder flux composition.
In still another particular embodiment, the solder flux composition comprises about 8 to about 16% phenylsuccinic acid, about 3.5 to about 5.0% ethylimidazole, about 3 to about 16% polyethylene glycol, about 60 to about 70% of at least one alkylene glycol butyl ether, and about 3 to about 15% thixotropic agent, wherein all percentages are percentages by weight, based on the total weight of the solder flux composition.
These and other aspects of the present disclosure are described in greater detail below.